The way to make the screen bigger on runescape is to go to the main screen of your computer (not runescape main) and right click on the screen then click properties.
This should make another screen pop up.
Now click on the tab that says settings you should come to another screen with a computer on it.
Now there should be a little box that says screen resolution now drag this to the left the further you drag it the bigger everything will get then click ok.
Now go to the runescape screen and it would be bigger.
Note: to change it back just drag it back to the size it was before.